Long fodder for maritime myth and pulp horror, the giant squid must be the most famous animal we still don’t understand very well, from its mating habits to the fundamental question of how many species it comprises.

Now, though, we know a little more.

According to a study published in Current Biology, the giant squid (and ‘colossal squid‘)’s huge eyes—which, aside from being the largest eyes of any known animal, are proportionally larger than those of other squid—have evolved for a special purpose:

…such giant eyes are unlikely to evolve for detecting mates or prey at long distance but are instead uniquely suited for detecting very large predators, such as sperm whales. […] we predict that, below 600 m depth, (the eye) would allow detection of sperm whales at distances exceeding 120 m […] we hypothesize that a well-prepared and powerful evasive response to hunting sperm whales may have driven the evolution of huge dimensions in both eyes and bodiesof giant and colossal squid.

Lead author Dan-Eric Nilsson, a marine vision expert at the University of Lund in Sweden, and his team determined that giant squid are lousy at seeing things up close—they’re farsighted—and good at seeing big things off in the distance. This offers a unique and much-needed advantage when your primary day-to-day threat to life and limb (and limb, and limb, and limb…ha!) is a 60-ton predator with nature’s most powerful sonar system.

But the boost in being able to sense contrast, which large eyes provide, is critical for detecting the low light differences of large, distant objects, the most important one being the bioluminescence stimulated by large animals such as approaching sperm whales, [report contributor and Duke biologist Sonke Johnsen] said.

The team realized that sperm whales dive and swim continuously while emitting sonar to ping the squid. The cephalopods are deaf to the sonar, but the whale’s wake triggers small organisms like plankton to produce light. Based on the design of the squid’s eye, the animal could see this light, though contrast is low, over “freakishly long distances,” about 120 meters — the length of an American football field, Johnsen said.

The giant squid’s massive eyes are (presumably) the latest stage in an epic arms race against sperm whales—a specific, complex adaptation that justifies their tremendous metabolic expenditure. The big eyes allow squid to spot sperm whales by their glowing wake from about 390 feet away, through murky expanses of deep ocean water, and escape—despite the predator’s sonar and speed.